Why do the Predators care about the PredAlien so much? I know it wiped out a few of them and is a monstrosity of a hybrid. However, why did they have it locked up on the ship? There is no way it got to full size (taking a few months I assume) or crawled around as a baby without a single Predator noticing. It must have been locked up, but if it's so bad, why keep it alive?
The Wiki literally explains this in the first two paragraphs.
I will paraphrase:
Scar, the "good Predator" from the previous movie, was carrying a chestburster that had both Alien and Predator genes. It bursts out of Scar, matures quickly and kills many Predators. The last surviving Predator manages to get a distress signal out to its homeworld, where a lone Predator takes it upon himself to come to Earth to kill all the Aliens.
It wasn't locked up, it was inside one of the Predator's bodies. Then it proceeded to kill all the Predators onboard the ship.